Touch sensor systems, such as touch screens or touch monitors, can act as input devices for interactive computer systems used for various applications, for example, information kiosks, order entry systems, video displays, mobile communications, etc. Such systems may be integrated into a computing device, thus providing interactive touch capable computing devices, including computers, electronic book readers, or mobile communications devices.
Generally, touch sensor systems enable the determination of a position on the surface of a substrate via a user's touch of the surface. The touch substrate is typically made of some form of glass which overlies a computer or computing device display, like a liquid crystal display (LCD), a cathode ray tube (CRT), plasma, etc. The touch sensor system is operatively connected to the device display so that it also enables the determination of a position on the device display and, moreover, of the appropriate control action of a user interface shown on the display.
Touch sensor systems may be implemented using different technologies. Acoustic touch sensors, such as ultrasonic touch sensors using surface acoustic waves, are currently one of the available touch sensor technologies and many types of acoustic touch sensors now exist. For example, an “Adler-type” acoustic touch sensor uses only two transducers per coordinate axis to spatially spread a transmitted surface acoustic wave signal and determines the touch surface coordinates by analyzing temporal aspects of a wave perturbation from a touch. For each axis, one transducer at a respective peripheral surface generates surface acoustic wave pulses that propagate through the substrate along a perpendicular peripheral surface along which a first reflective grating or array is disposed. The first reflective array is adapted to reflect portions of a surface acoustic wave perpendicularly across the substrate along plural parallel paths to a second reflective array disposed on the opposite peripheral surface. The second reflective array reflects the surface acoustic wave along the peripheral surface to a second transducer where the wave is received for processing. The reflective arrays associated with the X axis are perpendicular to the reflective arrays associated with the Y axis so as to provide a grid pattern to enable two-dimensional coordinates of a touch on the substrate to be determined. Touching the substrate surface at a point causes a loss of energy by the surface acoustic waves passing through the point of touch. This is manifested as an attenuation of the surface acoustic waves and is detected by the receiving transducers as a perturbation in the surface acoustic wave signal. A time delay analysis of the data is used to determine the surface coordinates of a touch on the substrate.
An acoustic touch sensor may have a large number of operative elements (either multiple transducers, or transducer and reflective array combinations) disposed on, and along, the front peripheral surfaces of the substrate. In order to prevent damage due to exposure from the environment or external objects, the housing for these sensors or for the devices integrating a sensor may include a bezel for the front peripheral surfaces of the touch substrate that hides and protects these peripheral operative elements, so that only an active touch region on the front surface of the substrate is exposed for possible touch input. For bezel-less acoustic touch sensors, the peripheral operative elements may be located on the back peripheral surfaces of the substrate (in this case, a surface acoustic wave propagates around a substrate rounded edge, across the front surface, and around the opposite substrate rounded edge to reach the receiving elements). Thin-width bezel and bezel-less acoustic touch sensors each enlarge the active touch region to essentially the whole front surface of the substrate, which may be beneficial for small-sized integrated devices, like a smartphone, a tablet computer, an electronic book reader, or other mobile computing device.
As the active touch region enlarges, more device features and touch functions may be provided in the active touch region. In some cases, however, these additional features and functions may interfere with the propagation of surface acoustic waves on the touch substrate. For example, in many bezel-less systems that have certain aesthetic considerations, the periphery of the back surface of the substrate may have an opaque ink or paint applied thereon with the peripheral operative elements, such as the reflective arrays and transducers, being printed on top of the “border paint” so that these elements are hidden from view through the typically transparent substrate. Further, a logo or application icon may be printed directly on the periphery of the back surface of the substrate underneath the border paint. This permits the printed logo or application icon to be seen through the substrate. Alternatively, the paint may be applied over (as described below) a cut out in the border paint having the shape of the logo or icon. However, a dip in the received signals at the receiving transducers as well as a sharp phase difference of the received surface acoustic waves may be observed at the location of the printed logo or application icon. In particular, observations show that there is a definite dip in the received signal and sharp phase differences in the received surface acoustic waves that correspond to the start of the printed material of the icon or application icon and again at the end of the printed material. Generally, it is believed that a surface acoustic wave experiences changes in velocity in passing over the printed material (and thus loses phase coherence) resulting in the observed effects. The paint layering and the likely, slower velocity of propagation of the printed material suggest reasons for the observed effects.
More specifically, it is believed that several types of reduced signal may result because of the printed material. In a first case that the printed material partially overlaps the reflective arrays, the signal downstream of the printed material slopes downward. This can be attributed to the printed material slightly redirecting the surface acoustic waves. In the second case that the printed material completely or mainly overlaps the reflective arrays, the signal downstream of the printed material is depressed. This can be attributed to the attenuation of horizontal surface acoustic waves going through the printed material paint. If the printed material extends below the reflective arrays, a dip in the received signal at the start of the printed material, because of the phase difference, is regularly observed. There should be another dip in the received signal at the end of the printed material, again due to the phase difference; however, this is not usually observed, possibly because it is masked. Also, if the printed material extends below the reflective arrays, the dip in the received signal will have the width of the printed material. This is due to the attenuation of the surface acoustic waves traveling in the vertical direction away from the reflective array.
It would be advantageous to have an improved acoustic touch apparatus that compensates for certain of the effects of surface acoustic wave velocity changes in passing over the printed material and averts reduced received signals that may result.